This invention relates to control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to an electronic fuel injection control system which is adapted to correct the air/fuel ratio in dependence upon atmospheric pressure and intake pipe absolute pressure so as to maintain the air/fuel ratio at an optimum value during exhaust gas recirculating operation.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Ser. No. 348,648, assigned to the same assignee as the present application, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc. by electronic computing means.
On the other hand, during operation of an engine at a high altitude, etc., it is generally carried out to correct the fuel supply quantity for the engine, in response to changes in the atmospheric pressure, so as to obtain an optimum air/fuel ratio best suited for the atmospheric pressure, for improvements in the fuel consumption, emission characteristics and driveability of the engine.
For instance, in a fuel supply control system adapted for correction of the basic valve opening period of a fuel injection valve by means of a correction coefficient as mentioned above, an atmospheric pressure-dependent correction coefficient is provided as one of the aforementioned correction coefficients, for correction of the air/fuel ratio of the mixture.
However, according to such conventional atmospheric pressure-dependent correction of the air/fuel ratio which is determined by intake pipe absolute pressure as noted above, the air/fuel ratio is corrected in dependence upon the atmospheric pressure alone. That is, the correction amount is not based upon the actual operating condition of the engine per se, making it difficult to perform the air/fuel ratio correction in a perfect manner.
On the other hand, in an engine which is provided with an exhaust gas recirculating device for improvement of the emission characteristics of the engine, absolute pressure in the exhaust gas recirculating passage at a location upstream of the exhaust gas recirculation valve, that is, back pressure in the exhaust pipe decreases with a decrease in the atmospheric pressure so that the exhaust gas recirculating rate decreases. As a consequence, the air/fuel ratio of the mixture becomes leaner. The degree of leaning of the air/fuel ratio is larger during exhaust gas recirculating operation than that when the exhaust gas recirculating operation is not effected.